In this description, an optical disc with a stack of N information layers (where N is an integer that is equal to or greater than two) will be referred to herein as an “N-layer disc”. Also, optical discs, each having multiple information layers, will be collectively referred to herein as “multilayer optical discs”. A “single-sided multilayer optical disc” refers herein to an optical disc, of which each of multiple information layers is irradiated with a light beam that has come through only one side of the optical disc.
In a multilayer optical disc, the distance as measured from its disc surface 100a, through which the incoming light enters the disc, to any of its information layers is sometimes called the “depth” of that information layer. Also provided between the shallowest information layer of a multilayer optical disc and its disc surface 100a is a transparent cover layer, which is often called a “light-transmissive layer”. Even though actually there is a light-transmitting layer between each pair of information layers, the distance from the disc surface 100a to an information layer of interest (i.e., the depth of that information layer) will be sometimes referred to herein as the “thickness of the light-transmissive layer”.
Also, in this description, the information layer that is located most distant from the disc surface 100a of an N-layer disc will be referred to herein as an “L1 layer”. And N information layers will be sequentially referred to herein as “L1 layer”, “L2 layer” . . . and “LN layer”, respectively, in the descending order of their distance from the disc surface 100a. In that case, in a four-layer disc, for example, the information layer that is located closest to (i.e., least distant from) the disc surface 100a is L4 layer.
In optical disc technologies, data can be read out from a rotating optical disc by irradiating the disc with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disc. On a read-only optical disc, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disc. On the other hand, on a rewritable optical disc, a recording material film, from/on which data can be read and written optically, is deposited by evaporation process, for example, on the surface of a substrate on which tracks with spiral lands or grooves are arranged. In writing data on a rewritable optical disc, data is written there by irradiating the optical disc with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
In a multilayer optical disc, when data is going to be read from, or written on, one of its multiple information layers stacked there (which will be referred to herein as “Layer A”), the light beam is focused on that Layer A. And when it is necessary to read or write data from/on another one of the information layers (which will be referred to herein as “Layer B”), the light beam is focused on that Layer B instead. To shift the focus position of the light beam from one information layer (i.e., a current layer) to another information layer (i.e., a target layer) in this manner will be referred to herein as a “layer change” or a “focus jump”.
The focus position of a light beam can be shifted in the depth direction of the information layers (i.e., in the thickness direction of the optical disc) by a focus actuator in an optical pickup, which includes a laser light source that emits the light beam, an objective lens to converge that light beam, and actuators to move the objective lens. The actuators include a tracking actuator for moving the objective lens in the radial direction of the optical disc and a focus actuator for moving the objective lens in the thickness direction of the optical disc.
To get a focus jump (which will also be referred to herein as a “layer-to-layer jump”) done, the focus position of the light beam needs to be shifted to its target position as quickly as possible by the focus actuator. For example, Patent Document No. 1 discloses a conventional technique for making such a focus jump on a dual-layer DVD or on a dual-layer Blu-ray Disc (BD).
According to a focus jump method for a dual-layer BD as disclosed in Patent Document No. 1, first of all, a spherical aberration correcting mechanism starts to be moved, and then a focus actuator will start to move an objective lens in a predetermined amount of time, which is approximately a half as long as the time it takes to move the spherical aberration correcting mechanism from a position associated with a first storage layer to a position associated with a second storage layer. That is to say, the focus actuator starts moving the objective lens while the spherical aberration correcting mechanism is approaching the position associated with the second storage layer. That is why when the focus position reaches the second storage layer, the spherical aberration correcting mechanism will also be on the verge of arriving at the position associated with the second storage layer. Consequently, a focus servo operation can be done properly on the second storage layer. According to another focus jump method, the focus position starts to be moved when the spherical aberration correcting mechanism reaches roughly a midpoint between the respective positions associated with the first and second storage layers. Thus, a focus servo operation can be performed properly after the focus position has reached the second storage layer.
According to the technique disclosed in Patent Document No. 1, if a focus jump operation needs to be performed (i.e., if a focus position needs to be shifted) by jumping two or more layers at a time in a multilayer optical disc with three or more information layers, the spherical aberration correcting mechanism and the focus actuator are moved from their positions associated with the current storage layer to their positions associated with an adjacent storage layer and the focus is once set on the adjacent storage layer. Next, a focus jump operation is performed in the same way to shift the focus position to the next adjacent storage layer. And by performing such a layer-by-layer focus jump to the adjacent storage layer a predetermined number of times, the focus position can still be shifted from one layer to another without losing the stability of the focus servo operation even if two or more storage layers should be jumped to get to the end point.